1. Field of the Invention
The present invention relates to a method for diagnosing a tank ventilation system of an internal combustion engine which includes a turbocharger and a compressor. The present invention further relates to a computer program which carries out all steps of the method according to the present invention if it runs on a computer. Finally, the present invention relates to a computer program product which has program code and is stored on a machine-readable medium for carrying out the method according to the present invention if the program runs on a computer or a control unit.
2. Description of the Related Art
Legislation requires fuel vapors, which are generated in the fuel tank of a motor vehicle, in particular of a motor vehicle having a gasoline engine, as a result of outgassing, to be stored on an intermediate basis in order to supply them thereafter to the combustion process in an internal combustion engine. An activated carbon container is used for intermediate storage. However, this container must be purged while the motor vehicle is being driven due to its limited storage capacity. Fresh air is conducted through the activated carbon container for this purpose, the stored fuel vapors are desorbed, and the resulting fuel/air mixture is supplied to the combustion process in a targeted manner. This entire process is based on a pressure gradient between the surroundings and the intake passage of the internal combustion engine. Purging of the activated carbon container may thus only take place when such a pressure gradient is present between the surroundings and the intake passage. However, this pressure gradient exists only very conditionally in the lower partial load operation in supercharged internal combustion engines or an internal combustion engine having compressors. To be able to ensure sufficient regeneration of the activated carbon container in driving mode for these engine concepts as well, so-called Venturi systems were introduced, which are also able to generate a negative pressure during supercharged operation due to a bypass line in a Venturi nozzle, which may then be used for purging the activated carbon container.
Such a tank ventilation system is shown in FIG. 1 for a supercharged internal combustion engine. An activated carbon container 11 is connected to a tank venting valve 12. Two lines 13, 14 branch off tank venting valve 12 to introduce the fuel/air mixture into intake passage 16 of an internal combustion engine 17. First line 13 includes a non-return valve 131 and ends at a discharge point 132 between internal combustion engine 17 and a throttle valve 164 in intake passage 16. Second line 14 includes a non-return valve 141 and ends in Venturi nozzle 151. A connecting line 15, which ends at a second discharge point 152 in intake passage 16 between an air filter 161 and a turbocharger 162, extends from the Venturi nozzle. Between a charge air cooler 163 and throttle valve 164, intake passage 16 branches into a first part, which ends in Venturi nozzle 151, and a second part, which ends in internal combustion engine 17.
In the United States of America, legislation also requires the systems for regenerating activated carbon container 11 to be monitored for functional capability. In addition to tank venting valve 12, these systems also include the tubing, i.e., line system 13, 14, 15, which is used to transport the fuel/air mixture. As a result, defects within the entire system must be detected. The diagnostic function used for this purpose is based on a negative pressure which develops in the fuel tank when tank venting valve 12 is opened. If valve 12 cannot be opened or a defect exists in the line system, no negative pressure forms in the tank and the system is detected as being defective.
Since this diagnostic principle is based on negative pressure, only parts 13, 14 of the line system in which in fact a negative pressure is present are diagnosable. If a defect exists in the line system at connection 15 between Venturi nozzle 151 and discharge point 152, a negative pressure forms in Venturi nozzle 151, and thus also in the tank system, despite the defect. The system is thus not detected as being defective, even though a defect exists and the fuel/air mixture is given off to the surroundings as hydrocarbon emissions. Since discharge point 152 in intake passage 16 is located upstream from turbocharger 162, but downstream from an air mass flow sensor (hot film anemometer (HFM); not shown) of internal combustion engine 17, air which was not detected by the air mass flow sensor may additionally reach the air supply system of the internal combustion engine through a defective or detached connecting line 15. As a result, the design specification exists that connecting line 15 must be inseparably connected to Venturi nozzle 151 and discharge point 152 between air filter 161 and turbocharger 162 and must also be designed to be “indestructible.” These specifications may not always be possible to adhere to, among other things for packing reasons.